server, ui: only offer a working directory when a tool reads it (#26762)

The working directory chip showed up as soon as the server exposed any
builtin tool, so a server started with just get_datetime, or a user who
turned every filesystem tool off in the settings, still got a control
that nothing would read.

Tools now declare whether they resolve their paths and run against the
working directory, next to the write permission they already publish in
the /tools listing. The WebUI shows the chip and enables the /cwd
command only when at least one such tool is both served and left
enabled.

tools/ui/src/lib/constants/chat-commands.ts

@@ -8,7 +8,7 @@ interface ChatCommandsOptions {
 	/** Gates `/prompt`. */
 	hasPrompts: () => boolean;
 	/** Gates `/cwd`. */
-	hasBuiltinTools: () => boolean;
+	hasCwdTools: () => boolean;
 }
 
 /**
@@ -32,7 +32,7 @@ export function getChatCommands(options: ChatCommandsOptions): ChatFormCommand[]
 			description: SET_WORKING_DIRECTORY_LABEL,
 			keywords: ['current working directory'],
 			action: ChatFormCommandAction.CWD,
-			disabled: !options.hasBuiltinTools()
+			disabled: !options.hasCwdTools()
 		},
 		{
 			name: 'model',